EPITHELIAL DEVELOPMENT AND FUNCTION

My laboratory focuses on two different aspects of the biology of the fruit fly Drosophila melanogaster. With both projects, are goal is combine multiple experimental approaches--genetic, molecular, and physiological/behavioral--to understand as fully as possible the processes that we are studying. Descriptions of the two projects are given below.

Project 1: The adult lethal gene drop-dead

We are studying the adult lethal gene drop-dead (drd) which, when mutated, causes flies to die during the first week of adult life. The drd gene encodes a so-called NRF domain protein—a member of a family of integral membrane proteins with some homology to bacterial acyl transferases. No biochemical function has yet been determined for any eukaryotic NRF domain protein. While drd has previously been reported to cause neurodegeneration, we have found that the mutant flies also have many other phenotypes. First, they have a problem moving food through their guts, as they end up with a large amount of food stuck in the crop, which is a food storage organ (see figure below). Second, mutant female flies are sterile, due to a defect in the cross-linking of some of the proteins that make up the eggshell.

In our lab, we make use of a variety of different genetic and biochemical techniques to understand the function of drop-dead. Recently, we have found that drd expression is required during metamorphosis, but not during adulthood, for adult viability. Also, we find that expression in multiple tissues is required for survival, suggesting that there might be multiple “causes of death” when the drd mutant flies die. Ongoing projects in the lab include screening for genes that lie in the same metabolic pathway as drd, identifying the drd expression pattern, and studying the defect in eggshell formation in drd mutant females.

Distribution of food within the gut of wild-type and mutant flies. On the left is the dissected gut of a wild-type fly that was kept on blue food. Note that food is present in the midgut (Mid) and rectum (Rec) but not in the crop (Cr) or cardia (Car). In contrast, the gut from a drd mutant fly (right) has a large amount of food in the crop and some staining also in the cardia.

Project 2: Control of epithelial ion and water transport

Many of the important homeostatic processes that are necessary for life, such as regulation of water and ionic balance or the absorption of nutrients, are mediated by transport across epithelial cell layers. Precise control of such transport pathways is essential for an organism to adapt to a changing environment. In this project we are studying a model transport epithelium, the Malpighian tubules, which function as the fly’s kidney. The Malpighian tubules produce urine by transporting ions—primarily potassium and chloride—and water out of the hemolymph and into the tubule lumen, which is contiguous with the gut. The rate of urine secretion is controlled by many diuretic and antidiuretic factors in response to changes in the hydration and feeding state of the fly. We have discovered that the biogenic amine tyramine (which is the compound found is red wine and cheese that some have linked to migraines) acts as one of these diuretic factors (Blumenthal, 2003). Application of tyramine causes an increase in the conductance of chloride across the tubule and thus an increase in urine production. We also find that not only can the tubule respond to tyramine, it can also synthesize it from the amino acid tyrosine. As shown in the figure below, we have found that tyramine synthesis and reception occur in different cells in the tubule. Tyramine is synthesized in the principal cells from tyrosine by the enzyme tyrosine decarboxylase, encoded by the gene Tdc1. The tyramine then activates receptors on the stellate cells—primarily the receptor encoded by the gene tarot (tro, for TA Receptor Of the Tubule, aka CG7431). This is the first example of cell-cell communication in an insect Malpighian tubule. Current projects ongoing in the lab involve characterizing second messenger pathways that modulate the response of the tubule to tyramine and identifying other gene products necessary for the synthesis and release of tyramine.

Model of tyramine (TA) action in the Malpighian tubule. In our model, tyrosine is taken up into the principal cells of the tubule (white) and converted into tyramine by the enzyme tyrosine decarboxylase (TDC). The tyramine is released from the principal cells and binds to receptors on the stellate cells (green), stimulating an increase in chloride conductance and urine secretion.